1. Field of the Invention
This invention relates generally to a method for making magnetic random access memory (MRAM) element with small dimension and high quality for further making a complete high density and high yield MRAM.
2. Description of the Related Art
In recent years, MRAMs using the magnetoresistive effect of a ferromagnetic tunnel junctions or a magnetic tunnel junctions (MTJ) have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can also cope with high-speed reading and writing. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating tunnel barrier layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction. Corresponding to the parallel and anti-parallel magnetic states between the recording layer magnetization and the reference layer magnetization, the magnetic memory element has low and high electrical resistance states, respectively. Accordingly, a detection of the resistance allows a magnetoresistive element to provide information stored in the magnetic memory device.
Typically, MRAM devices are classified by different write methods. A traditional MRAM is a magnetic field-switched MRAM utilizing electric line currents to generate magnetic fields and switch the magnetization direction of the recording layer in a magnetoresistive element at their cross-point location during the programming write. A spin-transfer torque (or STT)-MRAM has a different write method utilizing electrons' spin momentum transfer. Specifically, the angular momentum of the spin-polarized electrons is transmitted to the electrons in the magnetic material serving as the magnetic recording layer. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. As the volume of the magnetic layer forming the recording layer is smaller, the injected spin-polarized current to write or switch can be also smaller.
Further, as in a so-called perpendicular spin-transfer torque magnetic random access memories (pSTT-MRAM), both of the two magnetization films have easy axis of magnetization in a direction perpendicular to the film plane due to their strong magnetic crystalline anisotropy (shape anisotropies are not used), and accordingly, the device shape can be made smaller than that of an in-plane magnetization type.
To record information or change resistance state, typically a recording current is provided by an associated circuitry to flow in the stacked direction of the magnetoresistive element, which is hereinafter referred to as a “vertical spin-transfer method.” Generally, constant-voltage recording is performed when recording is performed in a memory device accompanied by a resistance change. In a pSTT-MRAM, the majority of the applied voltage is acting on a thin oxide layer (tunnel barrier layer) which is about 10 angstroms thick, and if an excessive voltage is applied, the tunnel barrier breaks down. Even at a normal voltage operation, the tunnel barrier still could be broken if the tunnel layer is atomically rough from some points where the barrier gets thinner. More, even when the tunnel barrier does not immediately break down, if recording operations are repeated, the element may still become nonfunctional such that the resistance value changes (decreases) and information readout errors increase, making the element un-recordable. Furthermore, recording is not performed unless a sufficient voltage or sufficient spin current is applied. Accordingly, problems with insufficient recording arise before possible tunnel barrier breaks down.
In the mean time, since the switching current requirements reduce with decreasing MTJ element dimensions, pSTT-MRAM has the potential to scale nicely at the most advanced technology nodes. To make MRAM cells (FIG. 1) with small dimension, a better way is to build an MTJ element (130) directly on a via (100), connecting an associated circuitry in a semiconductor wafer, in between a bottom electrode (120) and a top electrode (140). However, due to the presence of an electrical conducting material such as Cu, the top surface of the via is usually atomically rough, which, in return, will result in a rough MgO barrier (135) and a bad magnetic tunneling. To avoid this morphological defect due to rough VIA, people usually build the MTJ cell away from the underneath via (FIG. 2). Unfortunately, such memory architecture will increase the dimension of the MRAM cell tremendously and make it impossible to increase memory density.